This invention relates to a process for the selective hydrogenation of acetylenic hydrocarbons of a hydrocarbon mixture containing diolefinic hydrocarbons.
The process for hydrocarbon conversion at high temperature, such as, for example, steam-cracking, produce unsaturated hydrocarbons such as, for example, ethylene, propylene, butadiene, butenes, together with hydrocarbons boiling in the gasoline range; the olefinic and diolefinic hydrocarbons having from 2 to 4 carbon atoms, obtained by this process, also contain a certain amount of acetylenic hydrocarbons. The content thereof varies according the severity of the conversion process but is always too low to make their separation and use as such in petrochemistry really attractive. However their presence in admixture with olefinic and diolefinic hydrocarbons makes it difficult or even impossible to use the latter in petrochemistry. This is, for example, the case of butadiene from which vinyl acetylene and butynes must be removed to the largest extent in order to make it suitable for elastomer production.
For removing the acetylenic hydrocarbons, various separation processes have been proposed, such as, for example, extractive distillation whereby hydrocarbons of different degrees of unsaturation can be separated. Thus, for example, in the case of the C.sub.4 cut, it is possible to isolate the saturated compounds with 4 carbon atoms, the butenes, the butadiene and the acetylenic compounds. However, in order to obtain the desired purity, these processes require costly apparatuses and result, in addition, in a loss of yield due to the fact that the resultant hydrocarbons cut, containing a high proportion of acetylenic compounds, cannot be used in petrochemistry.
Other processes have been proposed to avoid these disadvantages; most of them consist of selectively hydrogenating the acetylenic compounds contained in the charge, for example a raw steam-cracking C.sub.4 cut. A typical charge contains, by weight, from 20 to 60% of 1,3-butadiene, from 40 to 80% of butane, butenes and/or isobutene and from 0.1 to 0.5% of acetylenic compounds, comprising, for example, 0.05 to 0.2% of butyne and 0.1 to 0.4% of vinylacetylene. The French Pat. No. 2036991, for example, describes a process of this type.
The steady increase in the severity of steam-cracking (increase in the reaction temperature) resulted these last years in the production of cuts of much higher contents of acetylenic compounds, often of 1% or more by weight, for example, from 0.2 to 0.5% by weight of butynes and from 1 to 3% by weight of vinylacetylene. Such concentration of acetylenic hydrocarbons raise new problems for hydrogenation processes, in particular the problem of the quick deactivation of the catalyst due to formation and deposit of polymers and to the progressive dissolution of the active metals the rate of which is directly proportional to concentration of acetylenic hydrocarbons.
The problem of preparing stable catalysts for such reactions is not simple. For example, the French Pat. No. 1502462 shows that even in case of a charge containing a relatively low content acetylenic hydrocarbons, a catalyst of palladium on calcined alumina has already lost a large part of its activity and its selectivity after only 7 days of use. High palladium contents provide for a slight increase of the catalyst life time, but result in an excessively high cost of the catalyst.